Reverse osmosis facilities typically employ an array of high-pressure vessels, each in the form of a right circular cylindrical pipe up to about 20 feet in length. Each high-pressure vessel holds one or more membranes, depending on the length of the pressure vessel and membrane.
Membranes are made in a several configurations, with the two most common configurations being spiral-wound and a hollow-fiber. In effect, a typical membrane might be described as a cylindrical cartridge analogous to a roll of toilet paper.
The membrane essentially fills the high-pressure vessel and periodically must be removed for cleaning or replacement.